feat(rsc): add React Server Components and Next.js App Router compatibility

[ctrlplusb]

Summary

Ensure easy-peasy works correctly when imported in React Server Component environments (Next.js App Router, etc.).

Parent issue: #1004

Background

easy-peasy is fundamentally a client-side state management library. RSC compatibility means ensuring it doesn't break when imported in a server context, not making it work in server components. Currently, importing easy-peasy in a Next.js App Router project without 'use client' will fail because the hooks use useContext.

Changes

1. Add 'use client' directive

• Add 'use client' to src/index.js (main entry point)
• Ensures bundlers correctly mark the module as client-only

2. Optional server-only export

• Consider exposing easy-peasy/server with just createStore (no hooks, no context)
• Useful for server-side store initialization in RSC apps (e.g. pre-populating state before streaming)

Existing SSR safeguards (already in place)

• getBrowserStorage in persistence.js guards against window being undefined
• getServerSnapshot is set to store.getState in hooks — correct for SSR hydration

Testing

• Test importing easy-peasy in a minimal Next.js App Router setup
• Verify no breakage in existing SSR scenarios
• Verify server export works for store initialization without React hooks